Optical fiber transmission systems typically contain electronic repeaters to transmit the optical signal over long distances. Optical fiber amplifiers are emerging as an alternative way in which to amplify optical signals in these transmission systems. The erbium-doped amplifier, for example, amplifies optical signals in transmission systems such as ultra-long distance systems, large capacity CATV systems and optical soliton transmission systems. These systems transmit optical signals that have a wavelength that is within the 1.5 .mu.m region. Most terrestrial optical fiber transmission systems transmit optical signals in the 1.3 .mu.m region, however.
Optical fiber amplifiers do not amplify optical signals of every wavelength. The erbium-doped amplifiers described above are not suited for amplifying optical signals in the 1.3 .mu.m range. Amplifiers are therefore sought for transmission systems that amplify optical signals in the 1.3 .mu.m region. Praseodymium (Pr) doped fluoride amplifiers (PDFA) have been observed to amplify optical signals in the 1.3 .mu.m range.
PDFAs, and other optical signal amplifiers, are doped glass materials with certain optical properties. Praseodymium-doped fluorozirconate glass (ZBLAN), for example, has been observed to yield a large gain, about 30 dB, at 1.31 .mu.m. Shimzu, Makoto, et al., "28.3 dB Gain Pr-Doped Fluoride Fiber Amplifier Module Pumped by 1.017 .mu.m InGaAs-LDs, "Proc. OSC/IOOC, PD-12, San Jose, Calif., 1993. The Pr.sup.+3 is first excited so that a higher energy state is sufficiently populated. The relaxation of the Pr.sup.+3 from this higher energy state results in the photon emission necessary for the amplification of the 1.3 .mu.m range optical signal.
The relaxation transition of interest which the Pr.sup.+3 undergoes is from .sup.1 G.sub.4 to .sup.3 H.sub.5. The lifetime of Pr.sup.+3 in the .sup.1 G.sub.4 level is short, however. It is reported to be 110 microseconds in ZBLAN, and is limited by non-radiative decay. Ohishi, Y., et al., Optical Letters, 16:1747 (1991). Pedersen, B., et al., "Optimization of Pr.sup.3+ :ZBLAN fiber Amplifiers, " IEEE Photonics Technology Letters, 4(5):446 (May 1992) observe that the small signal gain increases with numerical aperture and pump power when the wavelength of the optical signal to the amplifier is 1310 nm (1.31 .mu.m). According to Pedersen et al., the pump power needed to get a 30 dB signal gain from the amplifier is 100 to 400 mW, depending on the numerical aperture of the amplifier fiber. These power levels are approximately an order of magnitude higher than the pump power levels for erbium doped amplifiers. These power levels are unacceptably high for some applications because they currently require high power lasers or diode arrays for pumping rather than a single diode. Pedersen et al. represent that the numerical aperture would have to be enlarged to an unacceptable amount in order to reduce the power consumption and still obtain adequate gain. Therefore, a lower power solution to optical amplification in the 1.3 .mu.m range is sought.